This invention generally relates to vibration damping means and more particularly to damping mechanisms for reducing wind induced oscillations and vibrations of tall, slender structures such as lightpoles, flagpoles, antennas and the like.
Modal vibrations of tubular poles and similar structures positioned in an ambient wind field can occur and cause undesirable noise and mechanical failure of the structure and the supports therefore. For example, such vibrations frequently result in fatigue failure of the pole or failure of fittings and couplings which attach the pole to a supporting structure. In slender structures such as antennas, lightpoles and flagpoles the vibrations are primarily induced by vortex shedding of the wind flow around the structure, which cause the structure to oscillate generally transverse to the wind direction. As the wind passes around an elongated structure small vortices are shed from the primary wind flow path and, for cylindrical structures, the vortex shedding assumes a regular alternating pattern with vortices shed first from one side and then the other side of the flow path. In the vicinity of the shedding the flow behavior generates an alternating force on the cylinder lateral to the relative direction of wind flow. These oscillations become self-excited when the frequency of the wind induced oscillations reaches one of the resonant modal frequencies of the cylindrical structure.
The frequency of vortex shedding for a fixed cylinder has been found empirically to be related to the Strouhal Number as follows: EQU S=fd/v or f=Sv/d
where
S=Strouhal Number PA1 f=frequency of vortex shedding (sec-1) PA1 d=diameter of the cylinder (ft.) PA1 v=wind velocity (ft./sec.)
Where laminar flow exists around a cylinder, S has been found to be equal to about 0.2 with most of the excitation energy concentrated at a single frequency. Upon transition of the air flow in the boundary layer from laminar to turbulent flow near the shoulder of the cylinder, experimental tests have shown that the Strouhal Number drops to about 0.12. However, as the turbulent flow increases, wherein for example RN=6346Vd, the energy spectrum becomes increasingly broad-banded so that a significant amount of energy is again evident at a Strouhal Number of about 0.2. Thus, the frequency of vortex shedding (f) is primarily affected by the wind velocity.
The vortex shedding force acting on an elongated structure for a particular wind speed can thus excite a resonant modal response in the structure which results in large amplitudes of self-excited structural vibrations. These self-excited vibrations normally occur when the vortex shedding frequency is substantially coincident with one of the modal frequencies of the structure. Also, due to a phenomena known as "lock on", the structure can be caused to vibrate at a modal frequency by a range of wind speeds and accompanying vortex shedding frequencies which are approximately equal to the modal frequency of the structure. Therefore, since the structure vibrates only at well defined modal frequencies for broad ranges of vortex shedding frequencies, the elongated structure can be efficiently dampened by providing damping elements which have primary resonance frequencies which are substantially equal to the modal frequencies of the structure.
Numerous prior approaches taken to overcome wind induced vibrations of pole structures have been met with varying degrees of success. For example, cables and guy wires have been attached to the poles at the points of maximum deflection for limiting the extent of such deflections. In other circumstances, damping devices have been attached to the pole structures for reducing the magnitude of the wind induced oscillations. One type of damping device which employs a viscous liquid damping means is generally disclosed in U.S. Pat. Nos. 3,245,177; 3,266,600; 3,310,138; and 3,382,629. Another type of damping device which utilizes shear and friction generated within an arrangement of alternating stiff plates and viscoelastic materials to dissipate vibrational movements of a structure is generally disclosed in U.S. Pat. Nos. 3,159,249; 3,174,589; and 3,314,502. Other types of damping devices are disclosed in U.S. Pat. Nos. 3,568,805; 3,612,222; and 3,826,340.